Liquid crystal display devices have been used in various information display terminals and video devices. The major operating system for the liquid crystal display device is the twisted nematic("TN") mode and the super twisted nematic ("STN") mode. Though they are commercially used in the market at present, the problems of narrow viewing angle remain unsolved.
An In-Plane Switching ("IPS") mode liquid crystal display has been suggested to solve foregoing problems.
As described in FIG. 1, a plurality of gate bus lines 11 are formed on a lower insulating substrate 10 along an x direction shown in the drawings. The gate bus lines 11 are parallel to each other. A plurality of data bus lines 15 are formed along an y direction which is substantially perpendicular to the x direction. Therefore a pixel region is defined. At this time, a pair of gate bus lines 11 and a pair of data bus lines 15 are shown in the drawing so as to define the pixel region. The gate bus line 11 and the data bus line 15 are insulated by a gate insulating layer(not shown).
A counter electrode 12, for example in the form of a rectangular frame, is formed within the pixel region and is disposed at the same plane with the gate bus line 11.
A pixel electrode 14 is formed at each pixel region where the counter electrode 12 is formed. The pixel electrode 14 consists of a web region 14a which divides the region surrounded by the rectangular frame shaped counter electrode 12 in the y direction, a first flange region 14b connected to one end of the web region 14a and simultaneously overlapped with the counter electrode 12 of the x direction, and a second flange region 14c which is parallel to the first flange region 14 and is connected to the other end of the web region 14a. Thus, the pixel electrode 14 appears like the letter "I". Herein, the counter electrode 12 and the pixel electrode 14 are made of opaque metal layers. To ensure an appropriate intensity of electric field, the widths of both the counter and pixel electrodes are preferably in the range of 10.about.20.mu.m.
The pixel electrode 14 and the counter electrode 12 are insulated from each other by a gate insulating layer(not shown).
A thin film transistor 16 is disposed at the intersection of the gate bus line 11 and the data bus line 12. This thin film transistor 16 includes a gate electrode extended from the gate bus line 11, a drain electrode extended from the data bus line 15, a source electrode extended from the pixel electrode 14 and a channel layer 17 formed on the upper portions of the gate electrode.
A storage capacitor(Cst) is disposed at the region where the counter electrode 12 and the pixel electrode 14 overlap.
Although not shown in FIG. 1, an upper substrate(not shown) equipped with a color filter(not shown) is disposed on the first substrate 10 opposite to each other with a selected distance. Herein, the distance between the upper substrate and lower substrate 10 is smaller than the distance between the counter electrode region in the y direction and the web region of the pixel electrode thereby forming an electric field which is parallel to the substrate surface. Further a liquid crystal layer(not shown) having a plurality of liquid crystal molecules is interposed between the upper substrate(not shown) and the lower substrate 10.
Also, on the resultant structure of the lower substrate and on an inner surface of the upper substrate are formed homogeneous alignment layers respectively. By the homogeneous alignment layer, in the absence of electric field between the counter electrode 12 and the pixel electrode 14, long axes of liquid crystal molecules 19 are arranged parallel to the substrate surface. Also, by the rubbing axis of the homogeneous alignment layer, the orientation direction of the molecules 19 is decided. The reference R in the drawings means the direction of rubbing axis for the homogeneous alignment layer formed on the lower substrate 10.
A first polarizing plate(not shown) is formed on the outer surface of the lower substrate 10 and a second polarizing plate(not shown) is formed on the outer surface of the upper substrate(not shown). Herein, the first polarizing plate is disposed to make its polarizing axis to be parallel to the P direction of the FIG. 1. That means, the directions of rubbing axis R and polarizing axis P are parallel each other. On the other hand, the polarizing axis of the second polarizing plate is substantially perpendicular to that of the first polarizing plate.
When a scanning signal is applied to the selected gate bus line 11 and a display signal is applied to the data bus line 15, the thin film transistor 16 disposed adjacent to the intersection of the gate bus line 11 and the data bus line 15 is turned on. Then the display signal of the data bus line 15 is transmitted to the pixel electrode 14 through the thin film transistor 16. Consequently, an electric field E is generated between the counter electrode 12, where a common signal is inputted, and the pixel electrode 14. At this time, as the direction of electric field E is referenced as x direction as described in the FIG. 1, it has a predetermined degree of angle with the rubbing axis.
Afterward, when no electric field is generated, the long axes of the liquid crystal molecules are arranged parallel to the substrate surface and parallel to the rubbing direction R. Therefore the light passing through the first polarizing plate and the liquid crystal layer is unable to pass the second polarizing plate, and the screen shows dark state.
On the other hand, when the electric field is generated, the long axes(or short axes) are rearranged parallel to the electric field. Therefore the incident light passing through the first polarizing plate and the liquid crystal layer, passes the second polarizing plate, and the screen shows white state.
At this time, the direction of the long axes of the liquid crystal molecules change according to the electric field, and the liquid crystal molecules themselves are arranged parallel to the substrate surface. Accordingly, the viewer can see the long axes of liquid crystal molecules from all directions, and the viewing angle characteristic is improved.
However, the IPS mode liquid crystal display as described above also includes the following problems.
It is well known that refractive anisotropy(or birefringence, {character pullout}n) occurs due to the difference in lengths of the long and the short axes. The refractive anisotropy {character pullout} also varies according to the observer's viewing directions. Therefore a selected color can be shown in the region where the polar angle is of 0 degree and the azimuth angle is in the range of degrees 0, 90, 180 and 270, even in the white state screen. This is regarded as color shift and a more detailed description thereof is attached with reference to equation 1. EQU T.apprxeq.T.sub.0 sin.sup.2 (2.chi.).multidot.sin.sup.2 (.pi..multidot..chi.nd/.lambda.) . . . equation 1
wherein, T: transmittance;
T.sub.0 : transmittance to the reference light; PA1 .chi.: angle between an optical axis of liquid crystal molecule and a polarizing axis of the polarizing plate; PA1 {character pullout}: birefringence; PA1 d: distance or gap between the upper and lower substrates(thickness of the liquid crystal layer); and PA1 .lambda.: wavelength of the incident light. PA1 an upper substrate and a lower substrate opposed and separated by selected distance; PA1 a liquid crystal layer including a plurality of liquid crystal molecules and interposed between inner surfaces of the upper and lower substrates; PA1 a first electrode formed on the inner surface of the lower substrate; and PA1 a second electrode formed on the inner surface of the lower substrate, wherein the first electrode and the second electrode form an electric field for driving the liquid crystal molecules; PA1 wherein in the absence of electric field between the first and second electrodes, the liquid crystal molecules are aligned such that their long axis are parallel to surfaces of the substrates in a first direction; PA1 wherein after a selected voltage is applied therebetween, first and second diagonal electric fields are simultaneously formed in a pixel, the two diagonal electric fields are formed to be symmetrical with respect to the first direction; PA1 wherein the first and second electrodes are made of transparent materials; PA1 wherein the distance between the first and second electrodes is shorter than the distance between the upper and lower substrates; PA1 wherein widths of the first and second electrodes are determined such that liquid crystal molecules overlying the two electrodes are driven by the electric field generated between the first and second electrodes. PA1 a liquid crystal layer including a plurality of liquid crystal molecules and interposed between inner surfaces of the upper and lower substrates; PA1 a gate bus line and a data bus line formed in the lower substrate in a matrix configuration thereby defining pixel regions; PA1 a counter electrode formed at each pixel region in the lower substrate and the counter electrode having a body of a rectangular frame shape; a first branch disposed parallel to the gate bus line, connecting lengthwise sides of the body and dividing a region surrounded by the body into a first space and a second space; and a plurality of second and third branches diverged from the body or the first branch toward the first and second spaces as diagonal lines respectively; PA1 a pixel electrode formed at each pixel region in the lower substrate, the pixel electrode forming an electric field together with the counter electrode, the pixel electrode having a first bar overlapped with one of those surfaces lengthwise sides of the body of the counter electrode and disposed parallel to the data bus line; a second bar diverged from the first bar and overlapped with the first branch of the counter electrode; a plurality of third and fourth bars diverged from the first and second bars toward the first and second spaces respectively as diagonal lines, wherein the third bar is interposed between the second branches and the fourth bar is interposed between the third branches; PA1 a switching means formed adjacent to an intersection of the gate bus line and the data bus line for transmitting a signal from the data bus line to the pixel electrode; and PA1 homogeneous alignment layers interposed between the lower substrate and the liquid crystal layer and between the upper substrate and the liquid crystal layer, wherein the counter electrode and the pixel electrode are formed in the lower substrate; PA1 wherein the homogeneous alignment layer formed at the lower substrate has a rubbing axis which is parallel to the gate bus line and the data bus line, and the homogeneous alignment layer formed at the upper substrate has a rubbing axis which is anti-parallel to the rubbing axis of the homogeneous alignment layer formed at the lower substrate; PA1 wherein the diagonal branches in the same space are disposed parallel to each other, and the second branch and the third bar in the first space make an angle .theta. with the first direction, the third branch and the fourth bar in the second space make an angle -.theta. with the first branch; PA1 wherein the counter and pixel electrodes are made of transparent materials; PA1 wherein the distance between the second branch of the counter electrode and the third bar of the pixel electrode, and the distance between the third branch of the counter electrode and the fourth bar of the pixel electrode are smaller than the distance between the upper and lower substrates; PA1 wherein widths of the second, third branches and the third, fourth bars are determined such that liquid crystal molecules overlying the diagonal branches are substantially driven by the electric field. PA1 an upper substrate and a lower substrate opposed one another and separated by a selected distance; PA1 a liquid crystal layer including a plurality of liquid crystal molecules, interposed between inner surfaces of the upper and lower substrates; PA1 a gate bus line and a data bus line formed in the lower substrate in a matrix configuration thereby defining pixel regions; PA1 a counter electrode formed at each of the pixel regions of the lower substrate and shaped as a rectangular plate; PA1 a pixel electrode formed at each pixel region of the lower substrate, the pixel electrode forming an electric field together with the pixel electrode, the pixel electrode having a first bar overlapped with the counter electrode and disposed parallel to the data bus line; a second bar diverging from the first bar and overlapped with the counter electrode, wherein the second bar divides the counter electrode region into a first space and a second space; a plurality of third and fourth bars diverged from the first and second bars toward the first and second spaces respectively as diagonal lines wherein the third bar is interposed between the second branches and the fourth bar is interposed between the third branches; PA1 a switching means formed at an intersection of the gate bus line and the data bus line for transmitting a signal from the data bus line to the pixel electrode; and PA1 homogeneous alignment layers interposed between the lower substrate and the liquid crystal layer and between the upper substrate and the liquid crystal layer, wherein the counter electrode and the pixel electrodes are formed in the lower substrate; PA1 wherein the homogeneous alignment layer formed at the lower substrate has a rubbing axis which is parallel to the gate bus line and the data bus line, and the homogeneous alignment layer formed at the upper substrate has a rubbing axis which is anti-parallel to the rubbing axis of the homogeneous alignment layer formed at the lower substrate; PA1 wherein the diagonal branches in the same space are disposed parallel to each other, and the second branch and the third bar in the first space make an angle .theta. with the first branch of the counterdectrode, the third branch and the fourth bar in the second space make an angle -.theta. with the first branch of the counter electode; PA1 wherein the counter and pixel electrodes are made of transparent materials; PA1 wherein the distance between the second branch of the counter electrode and the third bar of the pixel electrode, and the distance between the third branch of the counter electrode and the fourth bar of the pixel electrode are smaller than the distance between the upper and lower substrates; and PA1 wherein widths of the second, third branches and the third, fourth bars are determined such that liquid crystal molecules overlying the diagonal branches are substantially driven by the electric field.
So as to obtain the maximum transmittance T, the .chi. should be .pi./4 or the {character pullout}nd/.lambda. should be .pi./2 according to the equation 1. As the {character pullout}nd varies with the birefringence difference of the liquid crystal molecules depending on the viewing directions, the value of .lambda. varies so as to make {character pullout}d/.lambda. to be .pi./2. According to this condition, the color corresponding to the varied wavelength .lambda. appears in the screen.
Accordingly, as the value of {character pullout}n relatively decreases at the viewing directions "a" and "c" toward the short axes of the liquid crystal molecules, the wavelength of the incident light for obtaining the maximum transmittance relatively decreases. Consequently, a blue color having a shorter wavelength than a white color can be looked in the screen.
On the other hand, as the value of {character pullout}n relatively increases at the viewing directions "b" and "d" toward the short axes of the liquid crystal molecules, the wavelength of the incident light relatively increases. Consequently, a yellow color having a longer wavelength than the white color can be looked in the screen.
Deterioration is caused in the resolution of IPS mode liquid crystal display.
Since the counter electrode 12 and the pixel electrode 14 of the IPS mode liquid crystal display are made of opaque metal layers, an aperture area of the liquid crystal display decreases, and the transmittance thereof also decreases. In addition, so as to obtain an appropriate brightness, a backlight with high intensity must often be used and thus electrical consumption increases, which is often undesirable.
To solve these limitations, a counter electrode 12 and a pixel electrode 14 made of transparent material have been proposed. In such a liquid crystal liquid display the aperture ratio is often increased, but the transmittance is often not improved. To produce an in-plane electric field, the distance l between the electrodes 12 and 14 must often be set to be greater than the cell gap d. To obtain an appropriate intensity of the electric field, the electrodes 12 and 14 have relatively large dimension of width, for example, 10 to 20.mu.m.
However, if the electrodes have such a large dimension of width, the liquid crystal molecules positioned right above the upper surfaces of the electrodes 12 and 14 do not move thereby forming equipotential lines. As a result, since the liquid crystal molecules positioned right above the upper surfaces of the electrodes continue to hold an initial configuration even in the presence of the electric field, the transmittance is increased little.